The present application relates to lateral thyristors, and more particularly to lateral FET-controlled thyristors which can be turned on and turned off by a single gate connection.
Note that the points discussed below may reflect the hindsight gained from the disclosed inventions, and are not necessarily admitted to be prior art.
A thyristor is a four-layer solid-state structure which has long been an attractive candidate for high-current switches. Thyristors combine very high current ratings with very high voltage-withstand capabilities, which makes this class of devices the leading candidate for very high voltage switches and for handling very high power. For example, as of 2012, off-the-shelf packaged thyristors can withstand more than 10,000 Volts, and can switch more than 10 Megawatts of power in each unit. However, the basic thyristor structure cannot be turned off by just returning the “turn-on” terminal to 0 Volts, or even to a small negative voltage. Once it is turned on, it stays on for as long as it can draw a minimum holding current.
The basic thyristor structure can be thought of as a merged structure which combines a PNP bipolar transistor with an NPN bipolar transistor. Each of these bipolar transistors provides the base current of the other, so there is potentially a positive feedback relationship: the collector current in the NPN is the base current of the PNP, and the collector current of the PNP is the base current of the NPN. The gain of a bipolar transistor is normally stated as “beta” (β), which is the ratio of collector current to base current. In a thyristor, there will be positive feedback if the product of the two betas is greater than one (βNPN·βPNP>1). If this positive feedback relation is present, then, whenever the thyristor is turned ON, it will draw current up to the maximum the external terminals can supply, or until the bipolar devices reach saturation. This condition is often called “latchup” or “latching.”
When this basic thyristor is OFF, the junction between the n-base and p-base regions will be reverse biased, and this condition blocks conduction, (The anode will be connected to a voltage which is more positive than the cathode voltage.) A depletion region, with a width depending on the applied voltage, will be present on both sides of this pn-junction formed by the base of the NPN transistor and the base of the PNP transistor. The two other junctions will be forward biased, but no current flows (other than leakage), since the reverse biased junction is present.
When the thyristor is ON, conduction is as follows. (Note that current is carried by both electrons and holes flowing in opposite directions, but current in the conventional sense only flows in one direction.) Holes will pass from the p+ anode region through the n-base region into the p-base region, and thence into the n+ cathode (where they will typically recombine with the majority carriers, which in the n+ region are electrons). Since the holes have positive charge, their movement means that current (in the conventional sense) flows from the anode to the cathode. Similarly, electrons will pass from the n+ cathode region through the p-base region into the n-base region, and thence into the p+ anode (where they will typically recombine with the majority carriers, which in this region are holes). Since the electrons have negative charge, their movement means that current (in the conventional sense) is opposite to the physical movement of the electrons, i.e. current flows from the anode to the cathode. Since current is carried by both electrons and holes, this thyristor is a bipolar (or “minority carrier”) device, and operates quite differently than unipolar (or “majority carrier”) devices, such as field-effect transistors, where current flows because of the motion of only one carrier type.
When a thyristor has been turned ON, it is electrically analogous to a simple junction diode, but with a lower forward voltage drop than a junction diode.
Turn-on in a thyristor is usually simple, but turn-off is the more difficult challenge in this technology. Many attempts have been made to achieve a thyristor structure which can be turned on and off by a single control terminal. However, many of the proposed devices require large gate currents, and/or have very complex structures, and/or require positive and negative gate drive capability, and/or use separate terminals for turn-on and turn-off gates.
Issued U.S. Pat. No. 7,705,368 to Rodov and Akiyama, which is commonly owned with the present application, described a fundamentally new structure for a MOS-controlled thyristor (“MCT”). The main example described is a vertical device in which a trench gate on the n+ side, when turned on, creates a channel in adjacent p-body material, so that a population at the bottom of the trench is at the same potential as the n+ emitter. This population of electrons provides a “virtual emitter” which increases the gain of the NPN component, and allows the device to go into latchup easily. For turn-off, the same gate is driven negative, so that (preferably) electron flow, in the mesa between adjacent gate trenches, is pinched off. However, a mesa width of less than the Debye length is required for the gate to turn the Rodov et al. device OFF.
The thyristor devices of Rodov et al. have great advantages over IGBTs, but the Rodov et al. devices usually require that the gate voltage be pulsed positive to turn the device ON and negative to turn the device OFF. A power circuit used to control this type of thyristor device would therefore be quite different from the power circuit used to drive most IGBTs, in which the gate voltage is held constant while in the ON state, and returned to zero volts to turn the IGBT off.
An improvement on the Rodov et al. devices is presented in U.S. application Ser. No. 13/632,991 of Blanchard, Akiyama, and Tworzydlo, which is hereby incorporated by reference. This too is a “virtual emitter” device, and can use the same turn-on mechanisms as Rodov. However, the new teaching in this improvement is that the ungated four layer structure will not maintain latchup if the virtual emitter disappears (i.e. if the gate is no longer turned on). This behavior provides easier turn-off. Lifetime control and emitter-base shorts across the NPN transistor can be used to ensure that the bipolar gains are low enough to allow turn-off.
The above examples have been described as vertical devices, i.e. as devices where the current-carrying terminals are on opposite surfaces of the semiconductor die. Vertical devices are usually preferable at the highest voltage and current ratings, since the thickness of the semiconductor material can be adjusted to avoid breakdown at the rated voltage, and most of the area of the die will carry a high current density. However, vertical devices are difficult to integrate with other electronics.